Scientists have discovered the origin of Earth's first continents

Geologists have discovered a missing link in the enigmatic story of how the continents grew—a revised origin story that doesn't require plate tectonics or any external factors to explain their formation. Instead, the findings, published last week in Nature Communications, rely solely on internal geologic forces that occurred within oceanic plateaus that formed during the first few hundred million years of Earth's history.

A major hurdle in understanding how the continents formed during the Archean Eon (four to 2.5 billion years ago) is identifying the building blocks of Earth's early crust. The „new” Archean crust that formed during this period consisted of a very distinct association of three types of granitoid rocks – tonalite, trondjemite and granodiorite (TTG).

Understanding what creates TTGs and the magmas that form them is difficult because many geologic processes occurred between their initial melting and final crystallization. Previous researchers focused on the trace element composition of these rocks, hoping to find clues about TTG magmas and their source.

„We observed a specific set of trace elements that were unaffected by the changes and beautifully preserved signatures from the original magma that formed the new TTG crust,” said Dr. Matthijs Smit said. UBC) Department of Earth, Ocean and Atmospheric Sciences. „These elements allowed us to trace back the chemical changes that TTG magmas went through and trace melt compositions back to their initial state and source – most likely a type of gabbro.”

„Funnily enough, many people have these types of rocks as a kitchen countertop,” says Dr. Schmidt. „In a sense, many people prepare their dinner on the types of rock that are responsible for the formation of our modern continents.”

The Archean TTG crust is still part of the continents today. For example, in North America they form much of the Canadian interior between the Cordillera Mountains in the west and the Grenville and Appalachian Mountains in the east. Most of Ontario, Quebec, Manitoba, Saskatchewan, the Northwest Territories, and Nunavut are composed of Archean crustal fragments dominated by TTGs and their slightly younger and more evolved granitic counterparts.

„All of these rocks—especially their composition—can be explained by the model we present,” Dr. Schmidt said. „Ours is a simple model in which TTGs and TTGs are typically associated younger rocks that may resemble oceanic plateaus as a result of slow burial, thickening, and melting of the precursor crust. The continental crust was destined to give way. As it continued to sink, the underlying rocks had no choice but to melt. In doing so, they created TTGs that proved a successful recipe for terrestrial survival and growth.”

The discovery by UBC researchers of a distinctly „intra-crustal” mechanism for the formation of TTGs debunks the long-held theory that Archean TTGs formed in Earth's first subduction zones and marked the beginning of plate tectonics.

„Always a 'chicken-and-egg' question came first—the initiation of plate tectonics or TTG magmatism to create new continental crust,” says Dr. Schmidt. „We show that these things may not actually be directly related. Recognizing the type of source rock makes this leap possible and explains the development of the first true continents from other mechanisms, such as meteorite impact.”

Dr. Smit and his UBC-based team's study used data from all TTG samples analyzed so far—samples of Archean cratonic fragments studied worldwide by researchers over the past 30 years. This is Dr. This allowed Smit and his team to filter out local anomalies and analytical problems, and to derive true trends in the composition of the rocks. The study used a large amount of data now available in the open source Geochemistry of the Oceans and Continents geochemistry database hosted by Georg-August-Universität, Göttingen.